Guidance on conveying carbon dioxide in pipelines in connection with carbon capture and storage projects
Introduction
Carbon Capture and Storage (CCS) projects are likely to involve the conveyance of Carbon Dioxide (CO2) by pipeline, either through the use of existing pipelines or the construction of new pipeline systems. Whilst some projects might involve conveying CO2 as a gas, it is likely that it will also be handled at high pressures as a dense or supercritical phase fluid.
Operators of CO2 pipelines need to fully understand the hazards, mechanisms, consequences and probabilities of pipeline failures in order to ensure their safety. Uncertainties remain around the conveyance of dense or supercritical phase CO2 in pipelines which are likely to be associated with CCS projects. HSE and the CCS sector are continuing to work to improve understanding of the risks from conveying CO2 in pipelines.
Legislation
Sections 2 and 3 of the Health and Safety at Work etc Act 1974 (HSWA) require employers to ensure the heath and safety of their employees and others so far as is reasonably practicable. This means that CO2 pipeline operators are required to take a proportionate approach to managing the risks from conveying CO2 at every stage of the pipeline's lifetime. This should be demonstrated through a comprehensive risk assessment which takes account of the range of risks that arise from the design, commissioning, operation (including maintenance and inspection) and decommissioning of the CO2 pipeline.
Part II of the Pipelines Safety Regulations 1996 (PSR) defines the legal standard for the design and operation of pipelines. In particular, Regulation 5 of PSR requires that the design of a pipeline, or any modification to it, takes account of:
- the operating regime of the pipeline
- the conditions under which the fluid is to be conveyed
- the environment to which the pipeline will be subjected.
Land Use Planning
CO2 is not currently defined as a dangerous fluid under PSR and, as such, CO2 pipelines are not classified as Major Accident Hazard Pipelines (MAHPs). Consequently developments around CO2 pipelines will not be subject to controls under Land Use Planning. However, HSE is committed to keeping under review the risks from conveying CO2 in pipelines and the classification of CO2 under major accident hazard legislation and will consider changes to existing legislation if it is justified by the evidence.
As Low as is Reasonably Practicable (ALARP)
Operators of CO2 pipelines can demonstrate compliance with PSR and HSWA by making sure that the risks from their pipelines are reduced as low as is reasonably practicable (ALARP). In particular, the application of good practice at the design stage is an essential part of this demonstration. Further information on ALARP and design can be found at Further guidance on assessing compliance with the law in individual cases and the use of good practice and Policy and guidance on Reducing Risks as Low as Reasonably Practicable in Design. However, to support their ALARP justifications, and until detailed standards become available, operators of CO2 pipelines should use sound engineering and empirical evidence to support un-validated or partially validated probabilistic modelling.
Conveying CO2 in pipelines
Conveying CO2 presents a number of hazards to the materials used to construct pipelines. Designers and operators should carefully consider the following:
Corrosion
CO2 is an acid gas which reacts with water to form carbonic acid. This means that water content of CO2 transported through carbon steel pipelines needs to be considered and pipeline materials selected accordingly. Other impurities in the CO2 stream should also be considered as sources of corrosion (when in reaction with water) and in some cases may present a greater hazard than carbonic acid.
Ductile and Brittle Fracture Propagation
Pipeline wall thickness and toughness should be specified to ensure that brittle fracture is avoided at normal operating temperatures and at those expected during a loss of containment event. It is also important so that ductile fracture is avoided or quickly arrested. These design considerations should be extended to all components in the pipeline system including welds, fittings etc.
Saturation Pressure
Where a dense phase or supercritical CO2 pipeline is ruptured, the concentration of impurities such as Nitrogen, Hydrogen, Oxygen and Argon will affect the saturation pressure of the released fluids. This is an important design issue because the time it takes the released CO2 to change from dense or supercritical phase to gaseous phase will have a marked effect on subsequent ductile crack propagation.
CO2 Stream Composition and Flow Assurance
Any pipeline or system of pipelines intended to convey CO2 should be designed and operated with regard to the range of impurities likely to be present in the CO2. Where CO2 from multiple sources is conveyed, each section of pipeline should be designed and operated to take account of the CO2 stream composition within that section. Also, given the likely variability of CO2 production, pipeline designers and operators should take account of the likely intermittency in flows and the consequent effects of repeated pressure cycling.
Modelling Loss of Containment
During a loss of containment event a number of highly complex interactions between the pipeline, the surrounding environment and the decompressing fluid will occur. With CO2 this is further complicated by the potential for phase changes of the fluid. This depends on the temperature and pressure as well as the geometry of the orifice through which the gas is decompressing and the presence and concentration of impurities. Given the complexity of modelling such a release, designers and operators should use existing outflow models which have been experimentally validated for use with CO2. Additionally, the use of computational fluid dynamics (CFD) to model releases and help set separation distances may be justified where a CO2 pipeline passes through a workplace, an occupied offshore installation or a populated area.
Non-Metallic Components
The presence of impurities in the CO2 stream may cause deterioration in non-metallic components such as elastomeric seals used in pipeline valves. As such, non-metallic components should be selected for use in CO2 pipelines only where their continued integrity in the presence of likely impurities has been demonstrated.
Fluid Hazard Classification
During a loss of containment event significant quantities of CO2 are likely to be released from a pipeline associated with CCS. Research published by HSE ('Comparison of risks from carbon dioxide and natural gas pipelines') concluded that a loss of containment event from a dense or supercritical phase CO2 pipeline presents a similar level of risk to a release from a high pressure natural gas pipeline. As such, designers of CO2 pipelines should consider applying a similar fluid hazard categorisation (chosen from an established pipeline design code) to that applied to high pressure natural gas pipelines.
Guidance and Standards
Current UK experience of designing and operating CO2 pipelines is limited and only some pipeline design codes include it as a relevant fluid within their scope. With regard to the re-use of existing pipelines, any proposal to change the fluid conveyed will require a re-assessment of the original pipeline design to ensure that the pipeline is capable of conveying the fluid safely. Oil and gas companies, particularly in the USA, do have some experience of using high pressure injection of CO2 in oilfields for enhanced oil recovery. However, the extent of the reliability data available from these activities is limited compared to that from hydrocarbon pipeline operation.
European Standards implemented in the UK as British Normative Standards (BS EN series) and supported by published documents (such as the British Standards PD series) provide a sound basis for the design of pipelines. Other national or international codes are likely to be acceptable in the UK if the proposed standard, code of practice, technical specification or procedure provides equivalent levels of safety.
UK and European standards relevant to the transport of CO2 in pipelines include:
- DNV RP-J202 – Design and Operation of CO2 Pipelines (2010)
- Energy Institute – 1301 CO2 Good Plant Design
- Energy Institute – 1302 CO2 Hazard Analysis
DNV RP-J202 and the Energy Institute guidance provide recommended practice on CO2 pipeline and plant design and operation and are intended to supplement other relevant standards.
UK and European Standards relevant to the general transport of fluids in pipelines include:
- BS PD 8010: 2004 Part 1 - Steel pipelines on land
- BS PD 8010: 2004 Part 2 - Subsea pipelines
- BS EN 14161: 2011 - Petroleum and Natural Gas Industries. Pipeline Transportation Systems
- Institute of Petroleum Pipeline Code IP6
- DNV OS-F101 - Submarine Pipeline Systems (2012)
BS PD 8010: 2004 Parts 1 and 2 and European Standard BS EN 14161 are general pipeline design codes of practice. HSE recommends that any pipelines designed to BS EN 14161 should be supported by industry good practice as presented in BS PD 8010: 2004 Parts 1 and 2.
Codes IP6, BS EN 14161, BS PD 8010 and DNV OS-F101 are all applicable to pipelines used to transport CO2. However none of these standards address CO2 transported in its dense or supercritical phases. Although IP6 is still in existence and contains useful guidance on operational issues, it is not widely used for new pipelines. DNV OS-F101 is specifically an offshore standard, limited to submarine pipeline systems.